RCS breeding tank?

You really dont need anything bigger than a 20 gal tank.
You dont need anything tall, so I would reccommend a 20 gal long tank.
Although I had more than 100 in a 10 gal tank without any issues.

keep it species only, nothing else in there. maybe a few snails, but nothing else.

What kind of substrate do you use?

I don't think it matters. For filtration either use a sponge filter or put a sponge over the intake of a HOB so the baby shrimp don't get sucked in.

I used a 5.5gal and started with 25. I had gravel substrate and java moss. when I hit 200 I moved them to a 10 after getting rid of 100.

I believe that Rachel had told me 10 per gallon ratio.

ditto the 20 long vs 20 high idea. Or 2-3 10G tanks. That way you have a spare in a tank crashes. Sand or gravel substrate they don't care. For best coloration I suggest a darker substrate.

I had them in a ~12G black storage box with a sponge filter and they did fine...but kinda hard to see 'em they colored up well in the black 'tank'.

In a clear storage box with sand, they were much lighter, even yellowish.

i prefer a thin layer of dark substrate. The general rule of thumb is 10 adults per gallon, but you can go over that if you are dilligent. I prefer 20 longs but I do use 29g and 30g as well for breeding tanks. You can comfortably keep many hundreds in those sizes. Sponge filter (hydro pro line) are my favorite. You may not need a heater if your house stays in the 70's. Do some low light plants or make sure you let the tank really establish. Feed VERY lightly, I feed 2x per week about 1 pellet per 10-20 shrimp. They will graze constantly on infusoria, biofilm, microalgaes, film algaes, etc and need very little supplementation if its an established tank.